1. Field of the Invention
The invention relates to a solid-state image pickup device and a method for manufacturing the solid-state image pickup device, and more particularly, to silicidation of a charge transfer electrode of a multilayer electrode structure.
2. Description of the Related Art
A CCD solid-state image pickup device used in an area sensor or the like has a photoelectric conversion section, such as a photodiode, and a charge transfer section equipped with a charge transfer electrode to be used for transferring a signal charge from the photoelectric conversion section. A plurality of charge transfer electrode are formed adjacent to a charge transfer path formed on a semiconductor substrate and are driven sequentially.
In the field of a solid-state image pickup device, an increase in the number of image pickup devices has recently been pursued up to the level of gigapixels or more. In association with an increase in the number of pixels, high-speed transfer of signal charges; i.e., driving of the charge transfer electrode by a high-speed pulse, is required, and hence demand exists for a decrease in the resistance of the charge transfer electrode. In the meantime, an increase in the size of a CCD sensor to, e.g., a Brownie (Trade Mark) size, is also pursued, and hence difficulty is encountered in maintaining high transfer efficiency at the time of transfer of electric charges.
Demand also exists for an increase in the transfer frequency of vertical CCDs with the view of preventing mixture of light into the vertical CCDs, to thereby reduce smear. To this end, a reduction in the resistance of the charge transfer electrode of the vertical CCDs is required in order to prevent occurrence of a false pulse and render a transfer pulse stable.
In response to these demands, related-art solid-state image pickup devices have adopted several methods; e.g., a method for increasing the thickness of a charge transfer electrode made of polycrystalline silicon, in order to diminish electrical resistance of the charge transfer electrode, and a method for increasing a doping level of dopants, such as phosphor or arsenic in polycrystalline silicon.
However, the thicker the film of the charge transfer electrode, the greater a step existing between the charge transfer electrode and a photoelectric conversion section such as a photodiode. Hence, an angle—at which a view of the light source is obtained from an aperture formed in the top of the photodiode—cannot be made wide, and therefore sufficient sensitivity fails to be attained.
A deterioration in flatness results in occurrence of variations in the thickness or geometry of various films such as a planarized film provided above the charge transfer electrode, an inner lens, a microlens, or a color filter. Consequently, shading, variations in sensitivity, and deterioration in smear due to stray light arise.
As mentioned above, demand for a reduction in the resistance of a film constituting the charge transfer electrode is ever-increasing. However, the resistivity of polycrystalline silicon to which dopants have been added up to the solubility limit assumes a value of 1000 μΩ cm or thereabouts, and hence a limitation is imposed on a reduction in resistance.
Therefore, the foregoing method encounters a problem of the difficulty in addressing a further increase in the number of pixels, an increase in the size of the solid-state image pickup device, and high-speed driving of the same.
To solve the problems, another proposed method for reducing electrical resistance of a charge transfer electrode is a structure in which a metal backing wire; that is, a so-called metal backing, is provided on a polycrystalline electrode (see JP-A-2001-223352).
When such a metal backing structure is used, a metal backing wire must be laid out with respect to the polycrystalline silicon electrode with an alignment margin. Hence, there arises a problem of a necessity for ensuring a wasted area corresponding to the alignment margin.
In the metal backing structure, a metal film, such as Al, Cu, or W, is used and connected to a charge transfer electrode after having been subjected to wiring processing. The density of defects, such as particles, in the metal film per se is higher than that of polycrystalline silicon. Therefore, the metal backing structure is likely to cause a problem in relation to yield, such as a short circuit in metal wiring.
When the metal film is subjected to patterning through photolithography during a process for subjecting the metal film to wiring, a step on the transfer electrode must inevitably be subjected to microprocessing. Hence, a variation in the width of a wiring pattern due to halation is likely to arise, raising a problem of difficulty in wiring.
A method for using a polycide structure in which polycrystalline silicon and silicide are stacked one on the other to reduce the resistance of the charge transfer electrode has already been proposed (see JP-A-63-46763).
After a pattern of charge transfer electrode has been formed through etching, the pattern is subjected to oxidation with the view of improving withstand voltage. A thermally-oxidated film of silicide constituting the polycide structure is lower in withstand voltage than the thermally-oxidated film formed by oxidizing polycrystalline silicon. Hence, there arises a problem of a reduction in withstand voltage between charge transfer electrodes, which in turn reduces yield.
To solve this problem, there has also been proposed a three-layered structure formed by containing a metal layer made of, for example Mo (molybdenum), in the polycrystalline silicon film. When pixel sizes have been made smaller, extreme difficulty is encountered in obtaining a metal-film-embraced structure without involvement of exposure of the metal layer from the polycrystalline silicon layer in all pixel areas and with superior reproducibility.
Even when an attempt is made to ensure a dielectric withstand voltage between transfer electrodes without involvement of oxidation in a polycide structure through use of a dielectric film CVD or the like, polycide is oxidized during a subsequent gate oxidated film formation process or during the course of heat processing to be performed at a high temperature (a temperature of 900° C. or higher) for activating dopants. Metal which constitutes diffused polycide and has a high melting point is captured into an oxide film existing between charge transfer electrodes, thereby deteriorating a dielectric withstand voltage.
There has also been proposed a method which is intended for ensuring a light-receiving area, reducing smear, and siliciding a charge transfer electrode through use of metal, such as molybdenum, after a polycrystalline silicon film has been patterned (see U.S. Pat. No. 5,202,282).
This methods enables a reduction in the resistance of the charge transfer electrode. However, this method is conceived with only a charge transfer electrode of single-layer structure in mind. There is a problem of inability to apply the method to a charge transfer electrode of two-layer structure.
In relation to the charge transfer electrode of multilayer structure, there is also conceivable a method for sequentially siliciding the electrode layers one after another. In relation to oxidation to be performed for improving withstand voltage between the charge transfer electrodes, a thermally-oxidated film to be formed has low dielectric withstand voltage as in the case of the previously-described charge transfer electrode of a polycide structure. Therefore, the method also suffers from a problem of an inability to improve yield.
Titanium silicide and cobalt silicide, which are used as an electrode material to be used for reducing resistance, have heat resistances of, at most, 700° C. to 850° C. or thereabouts. During the process for forming a gate oxide film of an upper electrode layer, the process of forming a charge transfer electrode, or the heating process for effecting activation during the process of forming a photodiode in a self-aligned manner through use of the ion implantation technique while the charge transfer electrode is taken as a mask, there arises a problem of occurrence of anomalous oxidation due to shortage of heat resistance, deterioration in a gate oxide film due to a coagulation reaction, or a hike in the resistance of a charge transfer electrode.
As mentioned above, a solid-state image pickup device in the related art has a problem of difficulty in reducing the resistance of a charge transfer electrode of a multilayer electrode structure and a problem of a reduction in yield associated with miniaturization and an increase in packing density.